The present invention relates to methods of geophysical exploration and more particularly to a method for determining the apparent or effective time break or firing time of a seismic source.
Basic geophysical exploration techniques are well-known. In general, some type of geophysical source is coupled to the earth so that, when it is activated, it initiates an acoustic wave which travels downward into the earth. Portions of the acoustic wave are reflected by subsurface interfaces and travel back towards to the earth's surface where they are detected by geophones, hydrophones or the like. The signals received from the geophone are recorded and processed by computers to produce seismic sections which represent the subsurface structure of the earth.
This exploration process can be improved if the precise firing time of the source is known. For example, the depth to the subsurface interfaces is determined by estimating acoustic velocity and measuring the travel time of acoustic waves from the source to the receiver. The travel time can be precisely measured only if the time of initiation is precisely known. In many modern exploration techniques, especially marine operations, linear or even two-dimensional arrays of sources are fired simultaneously to provide directivity to the acoustic wave and for other reasons. To achieve the results desired from such source arrays, it is essential that all of the sources actually be fired at the same time.
It is well-known that source firing time cannot be determined from the driving signal, usually electrical, used to fire or activate the seismic source. Every source has inherent time delays between receipt of the triggering signal and the actual initiation of the acoustic wave. As a result it is standard practice to equip seismic sources with detectors which respond to the actual acoustic wave generated. This time break signal is commonly recorded along with other data used in the various processing steps. This signal is also often used to determine the time delay between the firing signal and actual initiation of the acoustic wave. This time delay can then be used to adjust the firing signals to improve synchronization of sources which are fired as arrays.
While use of a source mounted detector to provide a time break signal provides improved results, it cannot avoid another source of error. In the region immediately surrounding the seismic source, the acoustic wave energy is usually so high that the wave travels in a non-linear manner. This region may extend only a distance of one foot from the source for small sources, but may extend as much as ten to twenty feet from the source for larger sources. Wave travel in the far field acoustic region beyond the non-linear region is uniform and predictable. Travel times measured from the time break signal received from a source mounted detector therefore include travel time through the non-linear region as well as the far field region. The non-linear velocities of the acoustic wave in the non-linear region are quite difficult to correct for, and introduce uncertainty as to the effective location of the source as seen from the far field region. Where arrays of multiple sources are used, variations in the non-linear region can time shift the effective firing location or time of the sources and thereby reduce the effectiveness of the source array. This is complicated further by the fact that source arrays often include sources of varying sizes each of which therefore has a different non-linear region even when they are all fired in a uniform medium such as sea water.